The present invention relates to a steel cord used to reinforce rubber articles, and a method of manufacturing the steel cord, and particularly to a steel cord used as a reinforcing material for a belt layer in a pneumatic tire, and a method of manufacturing the steel cord.
Further, the present invention also relates to a pneumatic radial tire in which a steel cord is used as a reinforcing material for a belt layer.
As a reinforcing material of a pneumatic radial tire, conventionally, a steel cord formed by twisting a plurality of steel filaments subjected to brass plating has been widely used.
Regions in which the steel cords are used as reinforcing materials of a radial tire are principally a carcass and a belt layer between the carcass and a tread rubber. The belt layer usually includes two or more layers of reinforcement plies. Each of the reinforcement plies is comprised of a large number of steel cords arranged in a transverse direction of the tire and embedded in rubber. Further, steel cords for each reinforcement ply are disposed at a predetermined angle of inclination to the equatorial plane of the tire and respective steel cords of adjacent reinforcement plies are disposed so as to cross each other, that is, so that angles of inclination thereof are formed in opposite directions with respect to the equatorial plane of the tire.
Usually, the steel cord is manufactured by twisting steel filaments in a helical manner by using a buncher-type or tubular-type wire twisting machine. Accordingly, the most common shape of a cross section of the steel cord is substantially circular. On the other hand, in order to improve penetration of rubber, improve tensile rigidity, and reduce the thickness of a belt layer, and the like, a steel cord having a flat cross section has been proposed. For example, an example of a steel cord having a layer twisted structure is given below.
Japanese Patent Application Laid-Open (JP-A) No. 9-156314 discloses a steel cord having a substantially elliptical cross section, which comprises a core having two filaments disposed in parallel and a sheath formed by twisting five to eight filaments around and close to the core. Further, JP-A No. 9-158965 discloses a steel cord having an elliptical cross section or having an elliptical or a substantially circular cross section depending on portions in a longitudinal direction of the steel cord, which steel cord is comprised of a core having a plurality of filaments transversely arranged in parallel without being twisted, and a sheath formed by twisting a plurality of filaments around and in externally contact with the core. Moreover, JP-A No. 9-158066 discloses a steel cord having a 2+6 structure, comprising a core having two filaments twisted at a twist pitch of 30 mm or more, in which the ratio of a minor axis to a major axis in any cross section of the cord is in the range from 100% to 68.4%.
On the other hand, there has recently been demanded for reduction in weight of steel cord required for each tire in order to achieve lightening of a tire from the standpoint of resources/energy saving or environmental protection.
However, when a weight of steel cord used in a belt layer of a tire is reduced, tensile rigidity in a tire circumferential direction of a belt layer which is a composite body of steel cord and rubber deteriorates. Accordingly, an amount of deformation of the belt layer during rolling of tires increases and a tread is apt to be worn down. In addition, rolling resistance increases and fuel consumption cannot be reduced. Further, it is not possible to achieve improvement in controlling stability, which has been demanded in association with the advance of vehicle performance. In order to reduce the weight of steel cord with the tensile strength of the steel cord being maintained at a sufficient level, it is necessary to use filaments having high tensile strength. For this reason, there have also been used, as a filament, a high-strength steel wire in which tensile strength T (N/mm2) and filament diameter d (mm) are set so as to satisfy the relation Txe2x89xa72250xe2x88x921450 logd, and an ultra high-strength steel wire which satisfies the relation Txe2x89xa72750xe2x88x921450 logd.
On the other hand, when a steel cord having a high strength per unit weight is used as a reinforcing material of a tire so as to only reduce the weight of the steel cord used, the above-described object cannot sufficiently be achieved.
Accordingly, in order that tensile rigidity of a belt layer which is a composite body of steel cord and rubber be improved, an angle of inclination of the steel cord in a belt layer, a distance between reinforcement plies, and the like have been designed in consideration of not only the tensile rigidity of steel cord, but also shear deformation resistance of rubber between steel cords. However, when a conventional layer twisted steel cord having a compressed cross section is used, the shear deformation resistance of rubber between steel cords cannot sufficiently be utilized. Therefore, it was difficult to obtain a belt layer whose weight is light and whose tensile rigidity is high.
In view of the above-described circumstances, it is a first object of the present invention to provide a steel cord in which tensile rigidity of the steel cord itself is high, and when the steel cord is used as a reinforcing material for a belt layer, shear deformation resistance of rubber between steel cords is effectively utilized, thereby allowing improvement in tensile rigidity of the belt layer per unit weight of steel cord used.
A second object of the present invention is to provide a method of manufacturing the steel cord adapted to achieve the first object of the present invention.
Further, a third object of the present invention is to provide a pneumatic radial tire in which the steel cord adapted to achieve the first aspect of the present invention is used as a reinforcing material for a belt layer, thereby allowing lightening of the tire, reduction in fuel consumption, and improvement in controlling stability.
In order to achieve the first object of the present invention, a steel cord comprising: a core; and an outer layer sheath including plural filaments, each filament having a diameter, all the diameters being substantially equal and said plural filaments being twisted around the core at substantially the same pitches, wherein said steel cord has a flat side surface portion on which at least two of said plural filaments forming the outer layer sheath are substantially arranged.
A conventional compressed layer twisted structure has a substantially elliptical cross section and has no flat side surface portion. On the other hand, the steel cord of the present invention has the above-described feature, and therefore, shear deformation resistance of rubber interposed between steel cords is effectively utilized. The reasons will be hereinafter explained.
Here, there is supposed a belt layer in which reinforcement plies formed by a large number of steel cords arranged in a transverse direction of a tire and embedded in rubber are laminated in a radial direction of the tire so that steel cords in adjacent reinforcement plies cross each other with rubber interposed therebetween. When a conventional steel cord having a substantially elliptical cross section is used as a reinforcing material, even if the steel cord is disposed so that the direction of a major axis thereof is placed parallel with a transverse direction of the reinforcement plies, respective bent side surfaces of adjacent steel cords in the direction in which the reinforcement plies are laminated face each other with rubber interposed therebetween. Accordingly, the thickness of rubber (hereinafter referred to as layer-to-layer rubber) interposed between the adjacent steel cords in the direction in which the reinforcement plies are laminated is not uniform. On the other hand, when the steel cord of the present invention is used as a reinforcing material and respective flat side surface portions of the adjacent steel cords in the direction in which the reinforcement plies are laminated are disposed so as to face each other with the layer-to-layer rubber interposed therebetween, the thickness of the layer-to-layer rubber interposed between the flat side surface portions becomes substantially uniform. Accordingly, when tensile force acts on a belt layer in which the steel cord of the present invention is used as a reinforcing material, the distribution of shear stress generated in the layer-to-layer rubber becomes more uniform. As a result, the layer-to-layer rubber can bear larger shear stress and a high tensile rigidity of an entire belt layer can be obtained. Further, concentration of shear stress generated in the layer-to-layer rubber is alleviated, and therefore, the durability of the belt layer also improves.
In order to allow uniform distribution of shear stress as described above, the width of the flat side surface portion is preferably large, and it is necessary that the flat side surface portion is one fourth or more, preferably a half or more of a cord width in a direction parallel with the flat side surface portion.
Further, it is effective that the flatness of the cord in the longitudinal direction thereof is high. Particularly, in side portions of the steel cord including the flat side surface portions facing each other with the layer-to-layer rubber interposed therebetween when the steel cord is used as a reinforcing material for the belt layer, the distance between adjacent filaments preferably does not exceed the diametrical dimension of the filament. More preferably, substantially no clearance is formed between adjacent filaments. To decrease the distance between the adjacent filaments in the flat side surface portion is effective also in improvement of tensile rigidity of the steel cord itself.
When a steel cord having a pair of flat side surface portions as periphery thereof facing parallel with each other with the core located therebetween is used for a belt layer in which a large number of reinforcement plies are laminated, the distribution of shear stress in the layer-to-layer rubber between adjacent reinforcement plies in a vertical direction can be made uniform.
Further, two pairs of flat side surface portions as periphery of the steel cord each facing parallel with each other with the core located therebetween can be provided. In this case, when the steel cord is used as a reinforcing material for a belt layer in such a manner that an angle formed by one of flat side surface portions belonging to the first pair and one of flat side surface portions belonging to the second pair becomes substantially a right angle, not only shear deformation resistance of the layer-to-layer rubber, but also shear deformation resistance of rubber between the adjacent steel cords provided in one reinforcement ply can also be utilized effectively.
In the steel cord of the present invention, the core preferably has a compressed cross section. The reason for this is that a clearance between the outer layer sheath for forming the flat side surface portions and the core can be made smaller, so as to allow improvement in rigidity of the cord itself and further facilitation of formation of the flat side surface portions. Particularly preferable is a steel cord which includes a core in which plural straight filaments are disposed parallel substantially on one plane and flat side surface portions substantially parallel to the one plane on which the filaments forming the core are disposed.
Further, in a steel cord in which a cord width a in a direction parallel to the plane on which the filaments for forming the core are disposed, and a cord width b in a direction perpendicular to the cord width a are given so as to satisfy the following expressions (1) and (2), the filaments or strands forming the outer layer sheath are twisted in close contact with the core. Accordingly, the tensile rigidity of the cord itself is high and the steel cord is particularly suitable to be used as a reinforcing material for a belt layer.
1.00xe2x89xa6a/(Xc+2xc3x97ds)xe2x89xa61.03xe2x80x83xe2x80x83(1)
1.00xe2x89xa6b/(Yc+2xc3x97ds)xe2x89xa61.03xe2x80x83xe2x80x83(2)
wherein, Xc is a large width of the core; Yc is a small width of the core; and ds is a diameter of each of the filaments forming the outer layer sheath.
Examples of the filaments forming the steel cord of the present invention include high strength steel wire whose tensile strength T (N/mm2) and filament diameter d (mm) are set so as to satisfy the relation Txe2x89xa72250xe2x88x921450 logd, and ultra high strength steel wire formed to satisfy the relation Txe2x89xa72750xe2x88x921450 logd. According to the present invention, even when the weight of steel cord used for each tire is reduced by using high strength steel wire or ultra high strength steel wire, a belt layer having sufficient tensile rigidity can be formed.
In order to achieve the second object of the present invention, a method of manufacturing a steel cord of the present invention relates to a method of manufacturing the steel cord adapted to achieve the first object. In a conventional method of manufacturing a steel cord having a compressed cross section, a steel cord having a substantially circular cross section manufactured by using a buncher-type or tubular-type wire twisting machine is compressed by a roller or the like to be made flat. However, since a steel cord whose cross section is originally substantially circular is made flat by compressing, if compressive force is released, the cross section tends to return to its original shape due to elasticity. For this reason, it is difficult to reliably manufacture a steel cord having flat side surface portions, which is suitably used in the present invention.
Accordingly, the method of manufacturing a steel cord of the present invention is characterized in that it includes the step of forming, around a core, an outer layer sheath formed by twisting plural filaments having the substantially same diameters at the same pitches, and further comprises the following steps.
(1) A step in which prior to formation of the outer layer sheath, at least two bending portions are formed in each of filaments forming the outer layer sheath per filament length equivalent to one twist pitch, with the filament length equivalent to one twist pitch being a cycle. The object of the step is to ensure the formation of the flat side surface portions by previously forming the bending portion at each of the portions of the filaments which portions should be located at ends of the flat surface portions when the cord is in the completed state.
(2) A step in which filaments are collected and twisted together so that bending portions belonging to the same phase of the steel cord are arranged substantially in one row along a cord axial direction on a cord side surface. The object of the process is to obtain a cord having a shape which is similar to the desired shape when filaments forming the outer layer sheath are twisted around the core. This step can be carried out by, for example, making a cross section of a hole formed in a twisting die into the substantially same shape as the cross section of the desired cord.
(3) A step in which the cord side surfaces are compressed. An object of the step is to increase the flatness of the side surface serving as the flat side surface portion to thereby obtain a desired steel cord. This step is preferably carried out by compressing the cord side surfaces simultaneously from four sides perpendicular to the cord side surfaces.
In order to achieve the third object of the present invention, there is provided, as a pneumatic radial tire of the present invention, a radial tire comprising: a pair of bead portions; a carcass extending between said bead portions in a toroidal shape; and a belt layer comprised of at least two layers of reinforcement plies in which steel cords are embedded parallel on a crown portion of said carcass, wherein said steel cords are each the steel cord according to the above-described present invention, and respective flat side surface portions of steel cords of adjacent reinforcement plies are disposed to face parallel with each other.